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Effect of lead nitrate on cyanidation of gold ores: progress on the study of the mechanisms
Pergamon
Minerals Engineering, Vol. 13, No. 12, pp. 1263-1279, 2000 © 2000 Minister of Public Works and Government Services Canada Published by Elsevier Science Ltd. All rights reserved 0892-6875(00)00109-6 0892-6875/00/$- see front matter
EFFECT OF LEAD NITRATE ON CYANIDATION OF GOLD ORES: PROGRESS ON THE STUDY OF THE MECHANISMS* G. DESCHENES ~, R. LASTRA ~, J.R. BROWN ~, S. JIN §, O. MAY ~ and E. GHALI § ~[Natural Resources Canada, CANMET, 555 Booth Street, Ottawa, K1A 0G1, Ontario, Canada Email: [email protected] § Department of Mining and Metallurgy, Laval University Ste-Foy, G1K 7P4, Quebec, Canada (Received 4 May 2000; accepted 10 July 2000)
ABSTRACT This paper discusses some of the latest efforts to improve the understanding of the use of lead nitrate in cyanidation. The study is based on an electrochemical approach to establish the nature of the mechanisms related to gold, a surface analysis study, using X-ray photoelectron spectroscopy (XPS), to determine the modifications on gold and sulphide minerals (pyrite, pyrrhotite and chalcopyrite) and an investigation that focus on the improvements of cyanidation. In a cyanide solutioi~, lead nitrate, lead sulphide and lead sulphite react with gold to form AuPb2, AuPb3 and metallic lead, which clearly accelerate the gold dissolution. The nature of the sulphide minerals affects the formation of lead or lead alloys on the gold surface. XPS did not find any lead on the surface of gold in the presence of pyrite or pyrrhotite but found a very thin layer (<50 ~) in presence of chalcopyrite. Further investigation is required to study the effect of the presence of other sulphides. It is proposed that in presence of sulphide minerals, sometimes lead does not report on gold because of its high affinity for sulphide minerals (competing reactions). Pyrite, chalcopyrite and pyrrhotite showed different reaction mechanisms with lead nitrate. Lead nitrate forms a hydroxide layer on pyrite particles, which reduces the reaction rate with cyanide. The dissolution of pyrite generates a sulphur layer on gold. This layer is less important in presence of lead. The effect of lead nitrate is subtler for chalcopyrite and pyrrhotite because it was less effective to retard the reaction of sulphides with cyanide and the reaction of iron with oxygen. For gold, the addition of lead nitrate has the same effect with pyrrhotite than with pyrite; inhibiting partially the formation of a sulphur layer. This was not observed for the gold in the chalcopyrite system. The results indicate that the strategy of lead nitrate addition, to be optimal, should be a function of the mineralogical composition of the ore. The formation of a passive layer on gold particles has a significant influence in the initial stages of leaching. The addition of lead nitrate notably decreased its inhibiting effect. In one case study, the lead nitrate treatment increased the overall gold extraction and decreased the cyanide consumption for an additional gross revenue of $CND2.2 millions. © 2000 Minister of Public Works and Government Services Canada. Published by Elsevier Science Ltd. All rights reserved.
* Presented at Hydromet 2000, Adelaide, Australia, April 2000
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Keywords Gold ores; sulphides; leaching; electrometallurgy; cyanidation
INTRODUCTION Poor cyanidation results are sometimes obtained when sulphide minerals are present in gold ores. The addition of lead nitrate to the slurry, with or without oxygen, enhances gold extraction and reduces cyanide consumption. This well-known practice is beneficial and has been used since the 1930's in cyanidation plants. Other additives (Hg, T1, and Bi) are reported to have similar effects (Fink and Putman 1950, Mclntyre and Peck 1976, Chimenos et al. 1997). Although it was suggested that lead is precipitated on gold as metallic lead (Rose and Newman 1898), the mechanisms involved were, until recently, at the hypothesis stage. The knowledge of the effect of lead nitrate on the surfaces of gold and sulphides is still fragmentary. Jeffrey et al. (1996) performed an electrochemistry study of the effect of lead on gold. Indirectly, by electrochemical observations, they inferred that lead on the surface of gold could occur as two types. The first form, believed to be lead metal, improved the gold dissolution rate. The second form was believed to be some kind of lead hydroxide film, which lowered the gold dissolution rate. The electrochemical study by Mussatti et al. (1997) was related to the aspects of the dissolution of gold in cyanide media containing lead. The gold surfaces were characterised by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDS). They found small "embryonic" crystals on the surface of gold. The EDS analysis of a single large (-10 pm) crystal indicated a Au-Pb compound. From the information of the large single crystal, they inferred that all the embryonic crystals were also Au-Pb. They also performed computer-aided thermodynamic calculations, which indicated that lead hydroxides are the lead species thermodynamically predominant when encountered during cyanidation conditions. They concluded that lead is deposited onto the gold surface and that the probable lead species is Pb(OH)3, resulting in the Au-Pb phase observed by SEM-EDS. Jeffrey et al. (1996) and Mussatti et al. (1997) suggested the formation of lead metal or lead oxide/hydroxide on gold. However, these possible mechanisms were not supported by experimental proof. Jin, et al. (1998), in fundamental electrochemical studies, also found the formation of lead and lead alloys on the surface of gold. The proposed mechanisms were supported by experimental results. The effect of sulphide minerals on the dissolution of gold was previously investigated by Liu and Yen (1995) using a wide variety of sulphide minerals (galena, arsenopyrite, pyrrhotite, pentlandite sphalerite, molybdenite, chalcopyrite, pyrite, chalcocite and stibnite). Their study focused on the use of oxygen to alleviate the negative effect of the sulphide minerals. The work concluded that the presence of some sulphides such as pentlandite arsenopyrite, pyrrhotite, sphalerite, molybdenite, chalcopyrite and pyrite increased the gold dissolution rate. Information gathered from cyanidation plants and cyanidation studies (Desch~nes and Fulton 1998, Desch~nes et al. 1999a and DeschEnes et al. 1999b) do not report any beneficial effect of these sulphide minerals on gold leaching kinetics. The exception is the beneficial effect of galena. CANMET started a consortium project (1995) to improve the industrial practice of the lead nitrate addition and to perform research to further understand the fundamental mechanisms of the effect of lead nitrate. The research program included: an investigation on the electrochemical behaviour of lead nitrate during cyanidation; a characterisation study to examine the alterations caused by the lead nitrate treatment on the surface of gold and the surface of the sulphides; and an investigation on the kinetics of cyanidation of gold ores in the presence of sulphides with and without lead nitrate additions, and the effect on reagent consumption.
Effect of lead nitrate on cyanidation of gold ores
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P A R T I - - E L E C T R O C H E M I C A L STUDY OF T H E E F F E C T OF LEAD ON G O L D In a previous report, it was found that the reaction between lead nitrate and gold in the presence of aqueous cyanide solution produced cubic crystals of metallic lead and tetragonal crystals of AuPb2 and AuPb3 alloys on the gold surface. It was the formation of these alloy phases that resulted in the potential drop of the gold electrode and accelerated the dissolution rate of the gold in the cyanide solution (Jin et al. 1998). In this paper, some results concerning the reaction between not only lead nitrate, but also sulphide and sulphite, and gold in cyanide solution are presented.
Experimental The working electrodes used in this study were short lengths of gold rod of 99.99% purity soldered to PVC insulated copper wires, cast in acrylic resin, ground with SiC abrasive paper down to 600 grit, rinsed with distilled water and swept with lint-free paper. The exposed area was 0.07 cm 2. The counter electrode was a platinized platinum foil. The reference electrode was mercurous sulfate (MSE); Hg, Hg2SOjsat.K2SO4 (0.64 V vs. NHE). A saturated K2SO 4 salt bridge was used to keep the reference electrode close to the working electrode. All potentials in this paper are given with respect to NHE. The electrolytic cell was a one liter glass cell containing 800 ml of electrolyte prepared from doubly-distilled water and stirred using a magnetic stirrer. The solution was purged with nitrogen. The pH was adjusted by adding NaOH. Sodium cyanide, sodium hydroxide and lead nitrate were A.C.S. reagent grade. All experiments were carried out at room temperature (23.0 _+ 0.5°C). The electrochemical measurements were conducted using an EG&G Princeton Applied Research 273 potentiostat/galvanostat controlled by an IBM computer and a software (SoftCorrTM).
Effect of metallic lead In order to prove that the anodic peak was related to lead precipitation on the gold surface, pure lead powders was pressed on the surface of a gold electrode using a spatula. Then the electrode was introduced into the NaCN solution for a potentiodynamic experiment. The obtained polarization curve is shown in Figure 1. The curve is similar to that where lead nitrate was added to the solution (Jin et al. 1998). Therefore, it is proposed that the anodic peak obtained when lead nitrate is added to the solution is related to lead precipitation on the gold surface. However, a pure lead electrode did not show that form of anodic peak, but gave a plateau in a more positive potential region as seen in Figure 2. The corrosion potential of the lead electrode in this solution was -357 mV, i.e., more positive than the gold-lead alloy phases. It is evident that the anodic peak produced from the gold electrode in NaCN solution with lead nitrate addition was due to neither gold nor lead alone, but to gold-lead alloys which have much more negative corrosion potentials and much higher corrosion rates than gold. 2000
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Polarization curve for gold electrode with pressed lead powders on the surface in 0.01 M NaCN solution bubbled by nitrogen and without lead addition. Other conditions: pH 11.1, room temperature (23°C), magnetic stirring, potential sweep rate: 1 mV/s.
1266
G. Deschenes
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POTENTIAL, mV vs. N H E Fig.2
Polarization curve for the lead electrode in 0.01 M NaCN solution without lead addition. Other conditions were the same as in Figure 1.
Effect of lead sulphide Similarly, the gold electrode surface was treated with lead sulphide powders, then the electrode was pickled with 30% HNO3 to remove the residual PbS. A potentiodynamic experiment was conducted using this electrode in the same 0.01 M NaCN solution as used for Figure 1, and the polarization curve shown in Figure 3 was obtained. The corrosion potential was - 505 mV vs NHE, and the corrosion current density was 56.9 ~tA/cm2 corresponding to a dissolution rate of 5.2 ~trn/day. This result has some practical interest for the gold industry. In gold production practice, abundant sulphides sometimes exist in the mineral pulp. When lead nitrate is added to the pulp, lead sulphide is formed. This does not mean that the lead salt looses its efficacy, but it can continue to affect the cyanidation process. The curve in Figure 3 has a comparable corrosion potential and a similar corrosion current to those for 10 ppm lead addition (as lead nitrate), -431 mV vs. NHE and 4.5 ~tA/cm2 respectively. This means that the effect of PbS on the cyanidation of gold has a similar intensity to that of lead nitrate. In addition, the polarization curve in Figure 3 shows a strong negative peak at 310 mV and small broad band around --400 mV. A further work is needed to explain these two things. 2000
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Polarization curve for gold electrode treated with PbS in 0.01 M NaCN solution. Other conditions were the same as in Figure 1.
Effect of lead nitrate on cyanidationof gold ores
1267
It is interesting to note that when the gold electrode treated with lead sulfide was put in 1 M NaCN solution without lead nitrate addition, and the same potential sweep used in Figure 3 was performed, four peaks were obtained as shown in Figure 4. This curve has the same feature as that obtained in 1 M NaCN with 100 ppm lead (in the form of PbNO3) using the same electrode (Jin et al. 1998). As interpreted by Jin et al. 1998, the peak between -600 and -500 mV corresponds to AuPb2 alloy, the peak between -500 and --400 mV corresponds to AuPb3 alloy, and the other two peaks correspond to the metallic lead phase. These three phases formed on gold surface were observed by SEM-EDS and confirmed by X-ray diffraction (Jin et al. 1998). It can be concluded that the lead sulphide accelerated the gold dissolution rate by the same mechanism as lead nitrate. 40 o4
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Polarization curve for gold electrode treated with PbS in 1 M NaCN solution. Other conditions were the same as in Figure 1.
Effect of lead sulphite Furthermore, it was noticed that the black lead sulphide could be very easily oxidized to white lead sulphite when it is exposed to air. So, the gold surface was treated with lead sulphite in the same manner as for the gold with PbS. The obtained polarization curve is shown Figure 5. The calculated corrosion potential is -400 mV VS. NHE, and the corrosion current density is 237 gA/cm 2 corresponding to a dissolution rate of 21.8 gm/day. Therefore, lead sulphide had a stronger activity for accelerating gold cyanidation rate. it can be concluded that insoluble lead compounds such as lead oxide, hydroxide, carbonate, sulphide and sulphite all can accelerate gold cyanidation rateThe condition is that the pulp must be well stirred to ensure a good contact between the lead salt and the gold particles.
P A R T 2---SURFACE A L T E R A T I O N S OF G O L D AND S U L P H I D E M I N E R A L S BY L E A D N I T R A T E IN CYANIDE S O L U T I O N S Experimental In broad terms, the method of investigation consisted of performing gold cyanidation experiments, with and without lead nitrate pre-leaching, in presence of a mixture of silica and the sulphide minerals. The surface of gold and sulphide minerals was analyzed by X-ray photoelectron spectroscopy (XPS). Three sets of cyanidation experiments were performed: cyanidation of gold in the presence of pyrite, in the presence of chalcopyrite and in the presence of pyrrhotite. A subset of these cyanidation experiments was
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G. Deschenes et al.
done with and without lead nitrate pre-leaching. The main feed materials for these cyanidation experiments were gold foils (99.99% Au) and sulphides minerals of high natural purity. The foils dimensions were -10 x 12 x 0.2 ram. A new gold foil was used for each cyanidation experiment. 3.0 O4
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Polarization curve for gold electrode treated with PbSO3 in 0.01 M NaCN. Other conditions were the same as in Figure 1.
The sulphide minerals used were characterized by chemical assay, X-ray diffraction analysis (XRD), image analysis (MP-SEM-IPS) and electron microprobe analysis (EMPA). Table 1 gives the chemical assay of these sulphide samples. XRD was performed to identify the minerals of major, medium and minor abundance. XRD spectra were obtained using a Rigaku 12 kW rotating anode X-ray powder diffractometer. The sulphide minerals used were of natural purity and XRD (Table 2) corroborated the major mineral. However, XRD also identified the presence of other sulphide minerals. A polished section was prepared of each sulphide and studied with an image analyzer to determine the mineral quantities in each sulphide. The system consists of a KONTRON IBAS image analyzer integrated with a JEOL 733 electron microprobe. Table 3 gives the mineral quantities in each of the sulphide feeds and shows that the "pyrite-rich" sulphide does have a high proportion of pyrite (96.6 wt%), therefore, it will be referred henceforth simply as "pyrite". The "chalcopyrite-rich" sulphide contains -60% chalcopyrite together with ~11% pyrite, -8% pyrrhotite, and ~10% sphalerite. Thus this sulphide sample is not near-pure chalcopyrite; it is proposed that the effect on cyanidation of this sulphide mixture is dominated by that of chalcopyrite, for simplicity it will be referred henceforth as "chalcopyrite", for detailed observations the amount of the different sulphides must be considered. Similar situation applies for the "pyrrhotite rich" sample which contains -72% pyrrhotite, - 11% pyrite, -2% arsenopyrite, - 1% chalcopyrite, - 1% galena, and 0.3% sphalerite. Thus this sulphide sample is not near-pure pyrrhotite; it is proposed that the effect on cyanidation of this sulphide mixture is dominated by that of pyrrhotite, for simplicity it will be referred henceforth as "pyrrhotite". Arnold (1967) studied the composition of natural terrestrial pyrrhotites from 82 different deposits in America. In general, he found that hexagonal pyrrhotite has an atomic % Fe above 47.5 whereas monoclinic pyrrhotite averages 46.5 atomic % Fe Elect~on microprobe analysis (EMPA) of one hundred randomly selected grains of the pyrrhotite sample was done to determine the population distribution of hexagonal and monoclinic pyrrhotite. The EMPA of all the pyrrhotite gains gave an atomic Fe% below 47.5; 94% of the analysed pyrrhotite grains clustered at 46.5+0.5 atomic % Fe; 6% of the analysed pyrrhotite grains gave results that indicated a mixture of hexagonal and monoclinic pyrrhotite. It is recognized that a slow XRD scan between -51 to -53 degrees can be used for quantitative analysis of monoclinic and hexagonal pyrrhotite. However, the EMPA data are sufficient in this case to indicate that a
Effectof lead nitrateon cyanidationof gold ores
1269
minimum of 94% of the pyrrhotite in the pyrrhotite-rich sample used in the present experiments is monoclinic pyrrhotite. Lehmann et al. (2000) have reported that the rate of dissolution in cyanide solution of monoclinic pyrrhotite under a variety of condition was greater than that of hexagonal pyrrhotite. Therefore, it is expected that monoclinic pyrrhotite has a stronger negative impact in cyanidation.
TABLE 1 Bulk chemical analysis [wt. %] for the sulphide minerals ELEMENT S Fe Pb As Zn Mg Si Ca Cu Ni A1 Co Sb Te Cd Ag Be
Pyrite-rich 51.1 46.3 0.39 0.13 0.1 0.048 0.032 0.03 0.023 0.022 0.016 0.0087 0.0063 0.0044 0.0035 0.0034 0.0016
Chalcopyrite-rich 37.0 28.9 0.015 0.2546 6.44 0.131 0.035 0.253 22.7 0.25 0.17 0.0277 0.0063 0.0100 0.0421 0.0376 0.0011
Pyrrhotite-rich 36.3 55.2 0.9 0.8 0.2 0.01 0.2 0.4 0.5 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
TABLE 2 Summary of X-ray diffraction analysis for the sulphide minerals
SAMPLE "Pyrite-rich" "Chalcopyrite-rich" "Pyrrhotite-rich"
MAJOR (>20%) Pyrite Chalcopyrite Pyrrhotite
Identified minerals other (<20%) galena, magnetite pyrite, sphalerite, siderite rhodochrosite, quartz, sphalerite
The cyanidation procedure began by addition of 400 ml of distilled water to a 500 ml reactor, starting the stirrer and then adding 200 g of silica-sulphide mixture. For the experiments with pyrite, the mixture was made with 90% quartz and 10% pyrite. For the experiments with chalcopyrite, the mixture was made of 95% quartz and 5% chalcopyrite. For the experiments with pyrrhotite the mixture was made with 90% quartz and 10% pyrrhotite. In all the experiments the silica was ground to 90% -75/am (200 mesh) and the sulphide feed was ground to 90% -300 ktm (48 mesh). Two gold foils were placed in the reactor. It is stressed that the experimental conditions for the pre-leaching and the cyanidation were not optimized for any of the sulphide minerals but were maintained constant to generate surface products that could be compared. For the experiments with lead nitrate pre-leaching, first the lime was added and after pH adjustment, lead nitrate was added. The pre-leaching was done for two hours under the following conditions: 8 ppm 02, pH 11.5, 100 g/t Pb(NO3)2. After the pre-leaching, one of the gold foils was extracted and washed with distilled water, then placed into a glass container and kept in a vacuum desiccator. This first set of gold foils was analyzed by XPS to determine the surface effect of the lead nitrate.
G. Deschenes et al.
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TABLE 3 Mineral quantities determined by image analysis in the used sulphide feeds MINERAL
Pyrite-rich
Quartz sio2
tr.
Silicates
1.7
Calcite CaCO3 Rhodochrosite MnCO3 Siderite FeCO3 Pyrite FeS2 Marcasite FeS2 Pyrrhotite ~-FeS Chalcopyrite CuFeS2 Covellite CuS Bornite CusFeS4 Tennantite (Cu, Fe)lzAS4S13 Sphalerite (Zn,Fe)S Arsenopyrite FeAsS Galena PbS
Chalcopyrite-rich
Pyrrhotite -rich 0.5
1.7 1.0 11.2
1.3 96.6 tr. 0.1 0.1
8.5 10.7
tr. 10.7
Tr. 7.8 60.1
72.2
tr.
Tr. 0.3 0.3 10.4
tr. 0.2
Tr.
1.4
0.3 1.7 1.0
tr. traces, less than 0.05% After the pre-leaching, cyanide was added to the reactor and cyanidation was performed for 4 hours under the following controlled conditions: 8 ppm 02, pH 11.5,500 g/t NaCN. During the cyanidation, a sample of the liquid was taken at 0.5, 1.0, 2.0 and 4 hours. The liquid samples were assayed for gold and free cyanide. At the end of the cyanidation period, the remaining gold foil was removed, washed gently with distilled water, and placed into a glass container and kept in a vacuum desiccator. This second set of gold foils was analyzed by XPS to see the surface effect of the lead nitrate followed by cyanidation. The leach residue was filtered to separate the solids, which were washed with distilled water. A 212 lain screen (65 mesh) was used to sieve out the coarser sulphides from the finer silica. The sulphide powder was stored in a glass container in a vacuum dessicator prior to surface analysis using XPS. A PHI-5600 small-spot XPS spectrometer equipped with two X-ray sources was employed to chemically characterise the surface of the specimens. One of the sources incorporates a monochromator (AI anode) and the second is an achromatic dual anode design (AI and Mg). For the present study, most spectra were collected using the Mg anode (1353.6 eV), however, in selected cases, the AI anode (1486.6 eV) was also employed to determine if particular element photolines (i.e., ones typically masked by Mg K,~ X-ray induced Auger lines) were present. XPS is capable of performing analysis of the outermost few atomic layers of solid samples, thereby providing surface analysis. Figures 6 to 12 give a summary of the XPS study in a comparative way. These Figures were prepared using selected elements considered relevant for the cyanidation experiments and for the pre-leaching with lead nitrate. The approximate analyses reported in these figures were calculated from surface analyses reported by XPS, but a small correction was applied to compensate for carbon contamination which comes either from the air, water and/or the vacuum grease in the desiccators. The carbon on the surface was adjusted to the carbon values found at 50 to 100 A. depths, whereas the proportion of the other elements at the surface was maintained constant. Despite this correction, the major elements on the surface are carbon and oxygen in carbon compounds from contamination. Because of this, it is important to note that the data in these Figures should be used in a comparative manner, for example sulphur on the surface of a given sulphide present in cyanidation with no pre-leaching compared with the experiment with pre-leaching.
Discussion The use of a gold foil was selected for analytical purposes. The consequence is that the passive layer related to the formation of reaction product on the substrate represents a smaller ration than it would be on gold
Effectof lead nitrateon cyanidationof gold ores
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grains. Major components of the surface analysis by XPS are gold (25-50%), carbon and oxygen from contamination with the aqueous or from manipulations (40-65%).
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Effect of lime and lead nitrate pre-leaching on the consumption of sodium cyanide in the cyanidation experiments with gold foil and in the presence of various sulphides. Pre-leaching conditions, where applicable: 8 ppm 02, pH 11.5, 100 g/t Pb(NO3)2, 2 hours. Cyanidation conditions: 8 ppm 02, pH 11.5,500 g/t NaCN, 4 hours.
The effect of lead nitrate on the surface of the sulphide is clearly displayed by the results with pyrite. Figure 7 shows that the predominant iron species at the surface of pyrite after the cyanidation with n o lead pre-leaching are as (Fe÷3) oxy-forms (i.e. as iron (3) oxides or hydroxides). Whereas, the predominant iron species at the surface of the pyrite after lead pre-leaching and cyanidation are as sulphides.
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Main differences of pyrite surfaces after cyanidation with no pre-leaching treatment and with lead nitrate pre-leaching treatment ( marked "Pb").
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The effect of lead nitrate on the pyrrhotite surface has some similarities with the case of chalcopyrite. Figure 8 shows that also the pyrrhotite surface both after cyanidation with no lead nitrate pre-leaching and pre-leaching followed by cyanidation have iron predominantly as oxy-forms species. The iron sulphide on the pyrrhotite surface after the lead pre-leaching is somewhat higher than on the pyrrhotite after cyanidation with n o pre-leaching. Also the pyrrhotite surface after the pre-leaching followed by cyanidation, contains lead hydroxide. This indicates that, as in the case of chalcopyrite, the pre-leaching is slowing down the surface reaction of the sulphur with the cyanide solution, but is not as effective in protecting the surface of pyrrhotite as in the case of pyrite. Figure 6, shows that, under identical pre-leaching, the effect is string in reducing the amount of cyanide used in presence of pyrite; whereas, this effect is not significant in the case of the cyanidation in the presence of chalcopyrite and pyrrhotite. XPS analyses were performed at the surface and at different depths up to 500 ~. As expected, the layers with lead on the pre-treated sulphides are not discrete; i.e. they do not end abruptly, but are diffuse. The maximum lead contents occur at the surface, smaller quantities of lead were found at depths of up to 500 A.. Pb
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I o.4 Pb-oxy forms
Main differences of pyrrhotite surfaces after cyanidation with no pre-leaching treatment and with lead nitrate pre-leaching treatment ( marked "Pb").
At the pre-leaching stage, part of the lead gets to the surface of the pyrite where it is found predominately as an oxy-form followed by lead sulphate. XPS, in practice, can not easily discriminate between the lead oxy-forms, (e.g. PbO, Pb(OH)2). However, other researchers (Mussatti e t al. 1997) have found by using computer-aided thermodynamic calculations that lead hydroxides are the species thermodynamically predominant at the conditions of the cyanidation. Therefore, the lead oxy-form referred in all the XPS tables must be the lead hydroxide Pb(OH)2. XPS found much less lead on the surface of the "fresh" pyrite and on the surface of the pyrite after cyanidation with n o pre-leaching. This lead is mainly as sulphate followed by lead hydroxide. The source of this lead is the small quantity of galena (-0.2 wt.%, Table 3) in the pyrite. The results indicate that lead hydroxide is formed on the surface of the pyrite. This yields an inhibiting layer at the pyrite surface, which slows down its reaction with cyanide. The lower concentration of sulphur at the surface of the pyrite, with no lead pre-leaching, indicates that sulphur reacted with cyanide. This reaction competes for the cyanide required for gold dissolution, increases the consumption of cyanide and reduces leaching kinetics. The effect of lead nitrate on the surface of chalcopyrite is subtler than in the case of pyrite. Figure 9 shows that the chalcopyrite surfaces both after cyanidation with no lead nitrate pre-leaching and pre-leaching followed by cyanidation have similar contents of iron oxy-forms. However, it clear that the surface of the chalcopyrite after lead pre-leaching and cyanidation contains more iron sulphides than the surface of the
Effect of lead nitrate on cyanidation of gold ores
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chalcopyrite after cyanidation with n o lead pre-leaching. It is interpreted that the pre-leaching, is less effective in inhibiting the reaction between chalcopyrite and cyanide than in the case of pyrite. Because in the case of chalcopyrite the predominant iron species is as oxy-forms. Whereas in the case of pyrite, the predominant iron species is as sulphides. This agrees well with the effect of the pre-leaching on the consumption of cyanide for these cyanidation experiments. Figure 6 shows that, under identical conditions, pre-leaching has a strong effect in reducing the amount of cyanide used in presence of pyrite; whereas, it is not efficient to reduce cyanide consumption for chalcopyrite. Probably a more aggressive pre-leaching would be required to have an effect.
20.0 S
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Pb
18.7
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Fe-oxy forms
Pb-oxy forms
Cu
Main differences of chalcopyrite surfaces after cyanidation with no pre-leaching treatment and with lead nitrate pre-leaching treatment ( marked "Pb").
Figures 10 to 12 give the approximate analysis for the surfaces of the gold foils. It is possible to observe that there is sulphur on the surface of most the gold foils. XPS indicated that the sulphur is present as sulphides or polysulphides of gold. The sulphur on the surface may passivate the gold and therefore could be detrimental for the cyanidation acting as a barrier. For the case of the cyanidation done in the presence of pyrite, there is more sulphur on the gold foils with n o lead pre-leaching than with pre-leaching (Figure I0). The lead nitrate pre-leaching reduces the dissolution of the sulphur from the pyrite. Some sulphur, instead of forming thiocyanate, is adsorbed on metallic gold. Therefore the less sulphide dissolution, the less sulphur adsorption occurs on gold. A similar phenomenon is seen in the case of the cyanidation in the presence of pyrrhotite (Figure 11). However, this is not observed for the case of chalcopyrite. In presence of lead addition, the gold foils from cyanidation with pyrite and chalcopyrite showed surface concentrations of iron, most of the time as iron oxy-forms. The iron content is lower at the outer most part of the layer than at 50 A depth, probably indicating a higher affinity of the iron for gold than for the sulphur containing part of the layer. The iron oxy-forms on the surface of the gold may be beneficial for the cyanidation. Only the gold foils from cyanidation in the presence of chalcopyrite after lead nitrate pre-leaching show a very thin layer (<50/~) containing lead, most probably as lead hydroxide. No lead was detected on the gold foils from cyanidation in the presence of pyrite and pyrrhotite. This indicates that lead has more affinity for pyrite and pyrrhotite than for the gold. No lead was detected on the gold foils from cyanidation in the
1274
G. Deschenes et al.
presence of pyrite and pyrrhotite. Further investigations under progress, using similar experiments, have confirmed that there is no lead on the gold foils from cyanidation experiments in the presence of pyrrhotite with lead nitrate pre-leaching but there is lead for the case where realgar is the sulphide present. In all, cases, it is clear that the formation of lead or lead alloys on the surface of the gold is not as strong and obvious as the case where gold is in contact with lead nitrate in cyanide solutions in the absence of sulphides.
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Main differences of gold surfaces after cyanidation in presence of pyrrhotite Lead nitrate preleaching treatment marked "Pb".
Effect of lead nitrate on cyanidationof gold ores
1275
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Main differences of gold surfaces after cyanidation in presence of chalcopyrite. Lead nitrate preleaching treatment marked "Pb".
PART 3---CYANIDATION STUDIES Experimental The ore samples were air-dried, homogenised and ground to 76%, 82%, and 90% -74~tm. The particle-size distribution used was similar to those typical of plant operation. The samples were cut into 500 or 1000 g lots using a rotary separator. Table 4 gives the assay for gold, silver and sulphur and the amount of cyanide consumers in the samples. The second sample has some unleachable gold, which is mostly in arsenopyrite. TABLE 4 Partial chemical and mineralogical analyses of the gold ores Ore Chemical analysis
Cyanicide (%)
1 4.21 0.90 1.53 0.4 3.1 tr.
Au (g/t) Ag (g/t) S (%) Pyrite Pyrrhotite Chalcopyrite Arsenopyrite tr. traces, less than 0.05%
2 4.63 0.63 1.29 2.7 tr. tr. 3.0
3 6.72 18.75 3.23 5.0 1.8 0.6 tr.
Lime, sodium cyanide, lead nitrate and oxygen were all certified reagent grade chemicals and plant water was used in the cyanidation tests. The gold leaching cells was made of glass and had a one litre or two litre capacity. The details of the experimental procedure are described in a previous report (Desch~nes and Fulton 1998). Air or a mixture of air and oxygen was injected to maintain the dissolved oxygen (DO) concentration constant, and the pH was controlled by lime addition. Water was added to maintain the pulp density at a constant value. In the case where lead nitrate was added during pre-leaching, it was introduced immediately after the start of mixing. No filtering was done after this stage and the cyanidation used the
1276
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same pulp. The cyanidation tests had duration of 24 h or 30 h. At the end of the experiment, the filter cake was washed with 1000 ml of distilled water. The cake wasdried, homogenized, sampled and analyzed for precious metals by fire assay. The leach solutions and wash solutions were titrated for free cyanide with silver nitrate and rhodamine as an indicator, and then assayed for gold and base metals (Fe, Cu) by atomic absorption spectroscopy. The calculation of cyanide consumption was based on total recycling of the leach solution. Table 5 presents the optimum experimental conditions and results for these cyanidation studies. The effect of the different leaching parameters studied is detailed in previous publications (Desch~nes and Fulton 1998, Desch~nes et al. 1999a, Desch~nes et al 1999b). Analysis of the data indicates that the lead nitrate addition strategy is a function of the mineralogical composition of the ore. The first gold ore, with only 0.4% pyrite and 3.1% pyrrhotite does not require a pre-leaching before cyanidation. Direct addition to cyanidation of a small amount of lead nitrate (50 g/t) reduced the cyanide consumed by 15% and increased the initial leaching kinetics (Figure 13). T A B L E 5 L e a c h i n g p a r a m e t e r s a n d p e r f o r m a n c e for the free m i l l i n g s u l p h i d e gold ores
Item \ Ore Grindin~ Pre-leaching
(% -74~tm) Duration (h) pH Dissolved oxygen (mg/L 02) Lead nitrate (~/t) Duration (h) pH Dissolved oxygen (m~/L 02) Lead nitrate (~/t) Free cyanide (ppm NaCN) Gold extraction (%) Tail (~/t Au) NaCN (kg/t)
Cyanidation
Efficiency
90
82
• --
76
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16
---
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Fig. 13 Effect of lead nitrate addition of the leaching of a gold ore with pyrite and pyrrhotite; Ore #1.
Effectof lead nitrateon cyanidationof gold ores
1277
The second gold ore, with 2.7% pyrite and 3.0% arsenopyrite, requires a very short pre-leaching time (between two and six hours) and 25-50 g/t lead nitrate. Improvement of the leaching kinetics resulted in a 43% reduction of the retention time. As illustrated in Figure 14, 89% gold extraction can be reached in 30 hours while the plant used previously 53 hours. At the plant scale (New Britannia Mine), the addition of lead nitrate also increased the overall gold extraction by 1.9%, from 89.2% to 91.1%. The cyanide consumption was reduced from 0.69 kg/t to 0.44 kg/t. The increment in gold recovered and the reduction in the cyanide consumption represented an additional gross revenue of $CND2.2 millions.
100 89%
90
50 ~ Pb(NO3)2
80
84%
70 ¢ 0
60 50
<
40 30 20 t0 0 0
6
12
18
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Fig. 14 Effect of lead nitrate addition of the leaching of a gold ore with arsenopyrite and pyrite; Ore #2. The mineralogical analysis of the third ore indicated 5.0% pyrite, 1.8% pyrrhotite and 0.6% chalcopyrite. Without oxygen enrichment of the pulp, a long pre-leaching period with lead nitrate was required because of the high concentration of chalcopyrite and to a lesser extent the high concentration of pyrite in the ore. The 16-hour pre-leaching period reduced the cyanide consumption by 38% (0.81 kg/t to 0.50 kg/t) and increased gold extraction by 4.2% (from 91.6% to 95.8%) for a 24 hour cyanidation. Figure 15 shows the effect of lead nitrate addition in the pre-treatment on the gold leaching kinetics. Modifications to the plant practice allowed for an annual additional gross profit of $CAN375,000 in 1998.
100 90
100 g/t Pb(NO3)z
80 70 60 50 <
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Effect of lead nitrate addition of the leaching of a gold ore with pyrite, pyrrhotite and chalcopyrite. Ore #3.
1278
G. Descheneset al.
Gold ore #3 has the highest cyanide consumption with 0.50 kg/t. The presence of chalcopyrite, 0.6%, is mainly responsible for the high reactivity of this ore with cyanide. Between 70-75% of the cyanide consumed is related to complexation with copper. Gold ore #2 is second for cyanide consumption with 0.33 kg/t. The second gold ore generated higher cyanide consumption than the first gold ore that has 3.1% pyrrhotite, 0.4% pyrite and a cyanide consumption of 0.22 kg/t. It was found in a previous study that pyrite is more detrimental than pyrrhotite for cyanide consumption (Desch~nes et al. 1999c). Particle size is also a factor in this case. The first gold ore was ground to 76% -74~tm compared to 90% - 7 4 p m for the second one. Therefore, fine grinding of a sulphide-containing ore increases cyanide consumption. The rate constant for the dissolution reaction for gold is a function of the formation of a passive layer. Figures 13-15 show that the passive layer has a significant influence in the initial stages of leaching. Crundwell and Godorr (1997) developed a model based on a shrinking particle equation. This model considers the electrochemical nature of the mechanism and the effect of a passive layer. However, to take into account the important influence of the passive layer in the initial stages of leaching, his equation would have to be modified. The critical concentration of free cyanide, which is defined as the minimum concentration of free cyanide to ensure an efficient extraction of gold, is influenced by the presence of the passive layer, in spite of the fact that lead nitrate diminishes its effect. Significant decrease of the leaching rates were observed when the free cyanide concentration was at 170 ppm, 180 ppm and 270 ppm NaCN for ores 1, 2 and 3 respectively. The real free cyanide concentration in the leach solution of the third ore is much lower than indicated because of the interference by copper cyanide during titration of free cyanide with silver nitrate. The fall in the gold extraction was also the highest for the 3rd ore with 30%.
CONCLUSIONS In a cyanide solution, lead nitrate, lead sulphide and lead sulphite react with gold to form AuPb2, AuPb3, and metallic lead and clearly accelerates the gold dissolution. The nature of the sulphide minerals affects the way that lead or lead alloys will be formed on the gold surface. XPS did not find any lead on the surface of gold in the presence of pyrite or pyrrhotite but found a very thin layer (<50 A) in presence of chalcopyrite. Further investigation is required to study the effect of the presence of other sulphides. In presence of sulphide minerals, sometimes lead does not report on gold because of its high affinity for sulphide minerals (competing reactions). Pyrite, chalcopyrite and pyrrhotite showed different reaction mechanisms with lead nitrate. Lead nitrate forms a hydroxide layer on pyrite particles which reduces the reaction rate with cyanide. The dissolution of pyrite generates a sulphur layer on gold. This layer is less important in presence of lead. The effect of lead nitrate is subtler for chalcopyrite and pyrrhotite because it was less effective to retard the reaction of sulphides with cyanide and the reaction of iron with oxygen. For gold, the addition of lead nitrate has the same effect with pyrrhotite than with pyrite; inhibiting partially the formation of the sulphur layer. This was not observed for the gold in the chalcopyrite system. The results of the electrochemical study and the study of sulphide minerals correlate well with the cyanidation case studies. The strategy of lead nitrate addition, to be optimal, should be a function of the mineralogical composition of the ore. For a gold ore with a low sulphide content, lead nitrate addition can be performed directly to the cyanidation. A gold ore with a higher sulphide content, with a low sulphide content but finely ground, or with a significant amount of copper sulphide minerals requires a pre-leaching with a lead nitrate addition. The formation of a passive layer on gold particles has a significant influence in the initial stages of leaching. The addition of lead nitrate notably decreased its inhibiting effect. Improvement of the leaching rate substantially increased the throughput. In one case study, the plant was able to increase its throughput 41%, increase its gold extraction by 4% and reduce its cyanide consumption by 37%. The additional gold extraction represents additional revenues of $CND1.9 Million and cyanide reduction is $CND0.32Million.
Effect of lead nitrate on cyanidation of gold ores
1279
ACKNOWLEDGEMENTS
The authors wish to thank CANMET for permission to publish this paper and Dr. Allen Pratt for his editorial comments. REFERENCES
Arnold, R.G., Range in composition and structure of 82 natural terrestrial pyrrhotites. The Canadian Mineralogist, 1967, 9, 31-50. Chimenos J.M., Segarra M., Guzman L., Karagueorguieva A. and Espiell F., Kinetics of the reaction of gold cyanidation in the presence of thallium(I) salt. Hydrometallurgy, 1997, 44, 269-286. Crundwell F.K. and Godorr, S.A, A mathematical model of leaching of gold in cyanide solutions. Hydrometallurgy, 1997, 44, 147-162. Desch~nes G. and Fulton, M., Improving cyanidation of a sulphide ore by using an efficient pre-leaching. In Proc. Int. Symposium on the Recovery of Gold. Montreal, CIM General Annual Meeting, May 1998 (in press). Desch~nes, G., Fulton, M. and Lafontaine, M., Assessment and control of the gold leaching parameters at Kiena Mines. In Proc. Symp. Control and Optimization in Minerals, Metals and Materials Processing, MetSoc/CIM, 38 th Ann. Conf. Metallurgists, 1999, 469-487. Desch~nes, G., Fulton, M., Jean P. and Healey, S. , Improvement of cyanidation at New Britannia Mine. In Proc. Randol Gold and Silver Forum'99, Denver. 1999, 149-154. Desch~nes, G., Rousseau, M., Tardif J., and Prud'homme, P.J.H., Effect of the composition of some sulphide minerals on cyanidation and use of lead nitrate and oxygen to alleviate their impact. Hydrometallurgy, 1999, 50, 205-221. Fink C.G. and Putman G.L., The action of sulphide ion and of metal salts on the dissolution of gold in cyanide solutions. Mining Engineering, Transactions AIME, 1950, 187,952-955. Jeffrey M.I., Ritchie I.M., and LaBrooy S.R. , The effect of lead on the electrochemistry of gold: myth or magic. Electrochemical Proceedings, 1996, 96--6, 284-295. Jin S., May Oliver, Ghali E. and DeschSnes G., Investigation on the mechanisms of the catalytical effect of lead salts on gold dissolution in cyanide solution. In Proc. Third International Conference on Hydrometallurgy, ICHM'98, (Yang Xianwan, Chen Qiyuan and He Aiping eds.), International Academic Publishers, Beijing, China,. 1998, 666-679. Lehmann M.N., Kaur P., Pennifold R.M. and Dunn J.G., A comparative study of the dissolution of hexagonal and monoclinic pyrrhotites in cyanide solution. Hydrometallurgy, 2000, 5, 255-273. Liu C.Q. and Yen, W.T., Effects of sulphide minerals and dissolved oxygen on the gold and silver dissolution in cyanide solution. 1995, Minerals Engineering, 8, 1/2, 111-123. Mclntyre J.D. and Peck W.F. Jr., Electrodeposition of gold - - depolarization effects induced by heavy metal ions. J. Electrochemical Society, 1976, 123, 1800-1813. Mussatti D., Mager J. and Martins G.P. Electrochemical aspects of the dissolution of gold cyanide electrolytes containing lead. Aqueous Electrotechnologies: Progress in Theory and Practice (D.B. Dreisinger, ed.), The Minerals, Metals & Materials Society, 1997, 248-265. Rose T.K. and Newman, W.A.C., The Metallurgy of Gold, 2 "a edition, C. Griffin and Cy Ltd, London. 1898.
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C o r r e s p o n d e n c e on papers published in Minerals Engineering is invited b y e-mail to bwills @ m i n - e n g . c o m
Minerals Engineering, Vol. 13, No. 12, pp. 1263-1279, 2000 © 2000 Minister of Public Works and Government Services Canada Published by Elsevier Science Ltd. All rights reserved 0892-6875(00)00109-6 0892-6875/00/$- see front matter
EFFECT OF LEAD NITRATE ON CYANIDATION OF GOLD ORES: PROGRESS ON THE STUDY OF THE MECHANISMS* G. DESCHENES ~, R. LASTRA ~, J.R. BROWN ~, S. JIN §, O. MAY ~ and E. GHALI § ~[Natural Resources Canada, CANMET, 555 Booth Street, Ottawa, K1A 0G1, Ontario, Canada Email: [email protected] § Department of Mining and Metallurgy, Laval University Ste-Foy, G1K 7P4, Quebec, Canada (Received 4 May 2000; accepted 10 July 2000)
ABSTRACT This paper discusses some of the latest efforts to improve the understanding of the use of lead nitrate in cyanidation. The study is based on an electrochemical approach to establish the nature of the mechanisms related to gold, a surface analysis study, using X-ray photoelectron spectroscopy (XPS), to determine the modifications on gold and sulphide minerals (pyrite, pyrrhotite and chalcopyrite) and an investigation that focus on the improvements of cyanidation. In a cyanide solutioi~, lead nitrate, lead sulphide and lead sulphite react with gold to form AuPb2, AuPb3 and metallic lead, which clearly accelerate the gold dissolution. The nature of the sulphide minerals affects the formation of lead or lead alloys on the gold surface. XPS did not find any lead on the surface of gold in the presence of pyrite or pyrrhotite but found a very thin layer (<50 ~) in presence of chalcopyrite. Further investigation is required to study the effect of the presence of other sulphides. It is proposed that in presence of sulphide minerals, sometimes lead does not report on gold because of its high affinity for sulphide minerals (competing reactions). Pyrite, chalcopyrite and pyrrhotite showed different reaction mechanisms with lead nitrate. Lead nitrate forms a hydroxide layer on pyrite particles, which reduces the reaction rate with cyanide. The dissolution of pyrite generates a sulphur layer on gold. This layer is less important in presence of lead. The effect of lead nitrate is subtler for chalcopyrite and pyrrhotite because it was less effective to retard the reaction of sulphides with cyanide and the reaction of iron with oxygen. For gold, the addition of lead nitrate has the same effect with pyrrhotite than with pyrite; inhibiting partially the formation of a sulphur layer. This was not observed for the gold in the chalcopyrite system. The results indicate that the strategy of lead nitrate addition, to be optimal, should be a function of the mineralogical composition of the ore. The formation of a passive layer on gold particles has a significant influence in the initial stages of leaching. The addition of lead nitrate notably decreased its inhibiting effect. In one case study, the lead nitrate treatment increased the overall gold extraction and decreased the cyanide consumption for an additional gross revenue of $CND2.2 millions. © 2000 Minister of Public Works and Government Services Canada. Published by Elsevier Science Ltd. All rights reserved.
* Presented at Hydromet 2000, Adelaide, Australia, April 2000
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Keywords Gold ores; sulphides; leaching; electrometallurgy; cyanidation
INTRODUCTION Poor cyanidation results are sometimes obtained when sulphide minerals are present in gold ores. The addition of lead nitrate to the slurry, with or without oxygen, enhances gold extraction and reduces cyanide consumption. This well-known practice is beneficial and has been used since the 1930's in cyanidation plants. Other additives (Hg, T1, and Bi) are reported to have similar effects (Fink and Putman 1950, Mclntyre and Peck 1976, Chimenos et al. 1997). Although it was suggested that lead is precipitated on gold as metallic lead (Rose and Newman 1898), the mechanisms involved were, until recently, at the hypothesis stage. The knowledge of the effect of lead nitrate on the surfaces of gold and sulphides is still fragmentary. Jeffrey et al. (1996) performed an electrochemistry study of the effect of lead on gold. Indirectly, by electrochemical observations, they inferred that lead on the surface of gold could occur as two types. The first form, believed to be lead metal, improved the gold dissolution rate. The second form was believed to be some kind of lead hydroxide film, which lowered the gold dissolution rate. The electrochemical study by Mussatti et al. (1997) was related to the aspects of the dissolution of gold in cyanide media containing lead. The gold surfaces were characterised by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDS). They found small "embryonic" crystals on the surface of gold. The EDS analysis of a single large (-10 pm) crystal indicated a Au-Pb compound. From the information of the large single crystal, they inferred that all the embryonic crystals were also Au-Pb. They also performed computer-aided thermodynamic calculations, which indicated that lead hydroxides are the lead species thermodynamically predominant when encountered during cyanidation conditions. They concluded that lead is deposited onto the gold surface and that the probable lead species is Pb(OH)3, resulting in the Au-Pb phase observed by SEM-EDS. Jeffrey et al. (1996) and Mussatti et al. (1997) suggested the formation of lead metal or lead oxide/hydroxide on gold. However, these possible mechanisms were not supported by experimental proof. Jin, et al. (1998), in fundamental electrochemical studies, also found the formation of lead and lead alloys on the surface of gold. The proposed mechanisms were supported by experimental results. The effect of sulphide minerals on the dissolution of gold was previously investigated by Liu and Yen (1995) using a wide variety of sulphide minerals (galena, arsenopyrite, pyrrhotite, pentlandite sphalerite, molybdenite, chalcopyrite, pyrite, chalcocite and stibnite). Their study focused on the use of oxygen to alleviate the negative effect of the sulphide minerals. The work concluded that the presence of some sulphides such as pentlandite arsenopyrite, pyrrhotite, sphalerite, molybdenite, chalcopyrite and pyrite increased the gold dissolution rate. Information gathered from cyanidation plants and cyanidation studies (Desch~nes and Fulton 1998, Desch~nes et al. 1999a and DeschEnes et al. 1999b) do not report any beneficial effect of these sulphide minerals on gold leaching kinetics. The exception is the beneficial effect of galena. CANMET started a consortium project (1995) to improve the industrial practice of the lead nitrate addition and to perform research to further understand the fundamental mechanisms of the effect of lead nitrate. The research program included: an investigation on the electrochemical behaviour of lead nitrate during cyanidation; a characterisation study to examine the alterations caused by the lead nitrate treatment on the surface of gold and the surface of the sulphides; and an investigation on the kinetics of cyanidation of gold ores in the presence of sulphides with and without lead nitrate additions, and the effect on reagent consumption.
Effect of lead nitrate on cyanidation of gold ores
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P A R T I - - E L E C T R O C H E M I C A L STUDY OF T H E E F F E C T OF LEAD ON G O L D In a previous report, it was found that the reaction between lead nitrate and gold in the presence of aqueous cyanide solution produced cubic crystals of metallic lead and tetragonal crystals of AuPb2 and AuPb3 alloys on the gold surface. It was the formation of these alloy phases that resulted in the potential drop of the gold electrode and accelerated the dissolution rate of the gold in the cyanide solution (Jin et al. 1998). In this paper, some results concerning the reaction between not only lead nitrate, but also sulphide and sulphite, and gold in cyanide solution are presented.
Experimental The working electrodes used in this study were short lengths of gold rod of 99.99% purity soldered to PVC insulated copper wires, cast in acrylic resin, ground with SiC abrasive paper down to 600 grit, rinsed with distilled water and swept with lint-free paper. The exposed area was 0.07 cm 2. The counter electrode was a platinized platinum foil. The reference electrode was mercurous sulfate (MSE); Hg, Hg2SOjsat.K2SO4 (0.64 V vs. NHE). A saturated K2SO 4 salt bridge was used to keep the reference electrode close to the working electrode. All potentials in this paper are given with respect to NHE. The electrolytic cell was a one liter glass cell containing 800 ml of electrolyte prepared from doubly-distilled water and stirred using a magnetic stirrer. The solution was purged with nitrogen. The pH was adjusted by adding NaOH. Sodium cyanide, sodium hydroxide and lead nitrate were A.C.S. reagent grade. All experiments were carried out at room temperature (23.0 _+ 0.5°C). The electrochemical measurements were conducted using an EG&G Princeton Applied Research 273 potentiostat/galvanostat controlled by an IBM computer and a software (SoftCorrTM).
Effect of metallic lead In order to prove that the anodic peak was related to lead precipitation on the gold surface, pure lead powders was pressed on the surface of a gold electrode using a spatula. Then the electrode was introduced into the NaCN solution for a potentiodynamic experiment. The obtained polarization curve is shown in Figure 1. The curve is similar to that where lead nitrate was added to the solution (Jin et al. 1998). Therefore, it is proposed that the anodic peak obtained when lead nitrate is added to the solution is related to lead precipitation on the gold surface. However, a pure lead electrode did not show that form of anodic peak, but gave a plateau in a more positive potential region as seen in Figure 2. The corrosion potential of the lead electrode in this solution was -357 mV, i.e., more positive than the gold-lead alloy phases. It is evident that the anodic peak produced from the gold electrode in NaCN solution with lead nitrate addition was due to neither gold nor lead alone, but to gold-lead alloys which have much more negative corrosion potentials and much higher corrosion rates than gold. 2000
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POTENTIAL, mV vs. NHE
Fig. 1
Polarization curve for gold electrode with pressed lead powders on the surface in 0.01 M NaCN solution bubbled by nitrogen and without lead addition. Other conditions: pH 11.1, room temperature (23°C), magnetic stirring, potential sweep rate: 1 mV/s.
1266
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POTENTIAL, mV vs. N H E Fig.2
Polarization curve for the lead electrode in 0.01 M NaCN solution without lead addition. Other conditions were the same as in Figure 1.
Effect of lead sulphide Similarly, the gold electrode surface was treated with lead sulphide powders, then the electrode was pickled with 30% HNO3 to remove the residual PbS. A potentiodynamic experiment was conducted using this electrode in the same 0.01 M NaCN solution as used for Figure 1, and the polarization curve shown in Figure 3 was obtained. The corrosion potential was - 505 mV vs NHE, and the corrosion current density was 56.9 ~tA/cm2 corresponding to a dissolution rate of 5.2 ~trn/day. This result has some practical interest for the gold industry. In gold production practice, abundant sulphides sometimes exist in the mineral pulp. When lead nitrate is added to the pulp, lead sulphide is formed. This does not mean that the lead salt looses its efficacy, but it can continue to affect the cyanidation process. The curve in Figure 3 has a comparable corrosion potential and a similar corrosion current to those for 10 ppm lead addition (as lead nitrate), -431 mV vs. NHE and 4.5 ~tA/cm2 respectively. This means that the effect of PbS on the cyanidation of gold has a similar intensity to that of lead nitrate. In addition, the polarization curve in Figure 3 shows a strong negative peak at 310 mV and small broad band around --400 mV. A further work is needed to explain these two things. 2000
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POTENTIAL, rnV vs. NHE
Fig.3
Polarization curve for gold electrode treated with PbS in 0.01 M NaCN solution. Other conditions were the same as in Figure 1.
Effect of lead nitrate on cyanidationof gold ores
1267
It is interesting to note that when the gold electrode treated with lead sulfide was put in 1 M NaCN solution without lead nitrate addition, and the same potential sweep used in Figure 3 was performed, four peaks were obtained as shown in Figure 4. This curve has the same feature as that obtained in 1 M NaCN with 100 ppm lead (in the form of PbNO3) using the same electrode (Jin et al. 1998). As interpreted by Jin et al. 1998, the peak between -600 and -500 mV corresponds to AuPb2 alloy, the peak between -500 and --400 mV corresponds to AuPb3 alloy, and the other two peaks correspond to the metallic lead phase. These three phases formed on gold surface were observed by SEM-EDS and confirmed by X-ray diffraction (Jin et al. 1998). It can be concluded that the lead sulphide accelerated the gold dissolution rate by the same mechanism as lead nitrate. 40 o4
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POTENTIAL, Fig.4
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mV vs, NHE
Polarization curve for gold electrode treated with PbS in 1 M NaCN solution. Other conditions were the same as in Figure 1.
Effect of lead sulphite Furthermore, it was noticed that the black lead sulphide could be very easily oxidized to white lead sulphite when it is exposed to air. So, the gold surface was treated with lead sulphite in the same manner as for the gold with PbS. The obtained polarization curve is shown Figure 5. The calculated corrosion potential is -400 mV VS. NHE, and the corrosion current density is 237 gA/cm 2 corresponding to a dissolution rate of 21.8 gm/day. Therefore, lead sulphide had a stronger activity for accelerating gold cyanidation rate. it can be concluded that insoluble lead compounds such as lead oxide, hydroxide, carbonate, sulphide and sulphite all can accelerate gold cyanidation rateThe condition is that the pulp must be well stirred to ensure a good contact between the lead salt and the gold particles.
P A R T 2---SURFACE A L T E R A T I O N S OF G O L D AND S U L P H I D E M I N E R A L S BY L E A D N I T R A T E IN CYANIDE S O L U T I O N S Experimental In broad terms, the method of investigation consisted of performing gold cyanidation experiments, with and without lead nitrate pre-leaching, in presence of a mixture of silica and the sulphide minerals. The surface of gold and sulphide minerals was analyzed by X-ray photoelectron spectroscopy (XPS). Three sets of cyanidation experiments were performed: cyanidation of gold in the presence of pyrite, in the presence of chalcopyrite and in the presence of pyrrhotite. A subset of these cyanidation experiments was
1268
G. Deschenes et al.
done with and without lead nitrate pre-leaching. The main feed materials for these cyanidation experiments were gold foils (99.99% Au) and sulphides minerals of high natural purity. The foils dimensions were -10 x 12 x 0.2 ram. A new gold foil was used for each cyanidation experiment. 3.0 O4
'
I
i
I
f
J
I
....... !
2.5
2.o Z UJ
1,5
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n,
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1.0 0.5
u
0.0
m
-0.5
m
-1.0 -700
I
I
I
-600
-500
-400
.....
POTENTIAL, Fig.5
I
I
I
I
-300
-200
-100
0
100
mV vs. NHE
Polarization curve for gold electrode treated with PbSO3 in 0.01 M NaCN. Other conditions were the same as in Figure 1.
The sulphide minerals used were characterized by chemical assay, X-ray diffraction analysis (XRD), image analysis (MP-SEM-IPS) and electron microprobe analysis (EMPA). Table 1 gives the chemical assay of these sulphide samples. XRD was performed to identify the minerals of major, medium and minor abundance. XRD spectra were obtained using a Rigaku 12 kW rotating anode X-ray powder diffractometer. The sulphide minerals used were of natural purity and XRD (Table 2) corroborated the major mineral. However, XRD also identified the presence of other sulphide minerals. A polished section was prepared of each sulphide and studied with an image analyzer to determine the mineral quantities in each sulphide. The system consists of a KONTRON IBAS image analyzer integrated with a JEOL 733 electron microprobe. Table 3 gives the mineral quantities in each of the sulphide feeds and shows that the "pyrite-rich" sulphide does have a high proportion of pyrite (96.6 wt%), therefore, it will be referred henceforth simply as "pyrite". The "chalcopyrite-rich" sulphide contains -60% chalcopyrite together with ~11% pyrite, -8% pyrrhotite, and ~10% sphalerite. Thus this sulphide sample is not near-pure chalcopyrite; it is proposed that the effect on cyanidation of this sulphide mixture is dominated by that of chalcopyrite, for simplicity it will be referred henceforth as "chalcopyrite", for detailed observations the amount of the different sulphides must be considered. Similar situation applies for the "pyrrhotite rich" sample which contains -72% pyrrhotite, - 11% pyrite, -2% arsenopyrite, - 1% chalcopyrite, - 1% galena, and 0.3% sphalerite. Thus this sulphide sample is not near-pure pyrrhotite; it is proposed that the effect on cyanidation of this sulphide mixture is dominated by that of pyrrhotite, for simplicity it will be referred henceforth as "pyrrhotite". Arnold (1967) studied the composition of natural terrestrial pyrrhotites from 82 different deposits in America. In general, he found that hexagonal pyrrhotite has an atomic % Fe above 47.5 whereas monoclinic pyrrhotite averages 46.5 atomic % Fe Elect~on microprobe analysis (EMPA) of one hundred randomly selected grains of the pyrrhotite sample was done to determine the population distribution of hexagonal and monoclinic pyrrhotite. The EMPA of all the pyrrhotite gains gave an atomic Fe% below 47.5; 94% of the analysed pyrrhotite grains clustered at 46.5+0.5 atomic % Fe; 6% of the analysed pyrrhotite grains gave results that indicated a mixture of hexagonal and monoclinic pyrrhotite. It is recognized that a slow XRD scan between -51 to -53 degrees can be used for quantitative analysis of monoclinic and hexagonal pyrrhotite. However, the EMPA data are sufficient in this case to indicate that a
Effectof lead nitrateon cyanidationof gold ores
1269
minimum of 94% of the pyrrhotite in the pyrrhotite-rich sample used in the present experiments is monoclinic pyrrhotite. Lehmann et al. (2000) have reported that the rate of dissolution in cyanide solution of monoclinic pyrrhotite under a variety of condition was greater than that of hexagonal pyrrhotite. Therefore, it is expected that monoclinic pyrrhotite has a stronger negative impact in cyanidation.
TABLE 1 Bulk chemical analysis [wt. %] for the sulphide minerals ELEMENT S Fe Pb As Zn Mg Si Ca Cu Ni A1 Co Sb Te Cd Ag Be
Pyrite-rich 51.1 46.3 0.39 0.13 0.1 0.048 0.032 0.03 0.023 0.022 0.016 0.0087 0.0063 0.0044 0.0035 0.0034 0.0016
Chalcopyrite-rich 37.0 28.9 0.015 0.2546 6.44 0.131 0.035 0.253 22.7 0.25 0.17 0.0277 0.0063 0.0100 0.0421 0.0376 0.0011
Pyrrhotite-rich 36.3 55.2 0.9 0.8 0.2 0.01 0.2 0.4 0.5 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
TABLE 2 Summary of X-ray diffraction analysis for the sulphide minerals
SAMPLE "Pyrite-rich" "Chalcopyrite-rich" "Pyrrhotite-rich"
MAJOR (>20%) Pyrite Chalcopyrite Pyrrhotite
Identified minerals other (<20%) galena, magnetite pyrite, sphalerite, siderite rhodochrosite, quartz, sphalerite
The cyanidation procedure began by addition of 400 ml of distilled water to a 500 ml reactor, starting the stirrer and then adding 200 g of silica-sulphide mixture. For the experiments with pyrite, the mixture was made with 90% quartz and 10% pyrite. For the experiments with chalcopyrite, the mixture was made of 95% quartz and 5% chalcopyrite. For the experiments with pyrrhotite the mixture was made with 90% quartz and 10% pyrrhotite. In all the experiments the silica was ground to 90% -75/am (200 mesh) and the sulphide feed was ground to 90% -300 ktm (48 mesh). Two gold foils were placed in the reactor. It is stressed that the experimental conditions for the pre-leaching and the cyanidation were not optimized for any of the sulphide minerals but were maintained constant to generate surface products that could be compared. For the experiments with lead nitrate pre-leaching, first the lime was added and after pH adjustment, lead nitrate was added. The pre-leaching was done for two hours under the following conditions: 8 ppm 02, pH 11.5, 100 g/t Pb(NO3)2. After the pre-leaching, one of the gold foils was extracted and washed with distilled water, then placed into a glass container and kept in a vacuum desiccator. This first set of gold foils was analyzed by XPS to determine the surface effect of the lead nitrate.
G. Deschenes et al.
1270
TABLE 3 Mineral quantities determined by image analysis in the used sulphide feeds MINERAL
Pyrite-rich
Quartz sio2
tr.
Silicates
1.7
Calcite CaCO3 Rhodochrosite MnCO3 Siderite FeCO3 Pyrite FeS2 Marcasite FeS2 Pyrrhotite ~-FeS Chalcopyrite CuFeS2 Covellite CuS Bornite CusFeS4 Tennantite (Cu, Fe)lzAS4S13 Sphalerite (Zn,Fe)S Arsenopyrite FeAsS Galena PbS
Chalcopyrite-rich
Pyrrhotite -rich 0.5
1.7 1.0 11.2
1.3 96.6 tr. 0.1 0.1
8.5 10.7
tr. 10.7
Tr. 7.8 60.1
72.2
tr.
Tr. 0.3 0.3 10.4
tr. 0.2
Tr.
1.4
0.3 1.7 1.0
tr. traces, less than 0.05% After the pre-leaching, cyanide was added to the reactor and cyanidation was performed for 4 hours under the following controlled conditions: 8 ppm 02, pH 11.5,500 g/t NaCN. During the cyanidation, a sample of the liquid was taken at 0.5, 1.0, 2.0 and 4 hours. The liquid samples were assayed for gold and free cyanide. At the end of the cyanidation period, the remaining gold foil was removed, washed gently with distilled water, and placed into a glass container and kept in a vacuum desiccator. This second set of gold foils was analyzed by XPS to see the surface effect of the lead nitrate followed by cyanidation. The leach residue was filtered to separate the solids, which were washed with distilled water. A 212 lain screen (65 mesh) was used to sieve out the coarser sulphides from the finer silica. The sulphide powder was stored in a glass container in a vacuum dessicator prior to surface analysis using XPS. A PHI-5600 small-spot XPS spectrometer equipped with two X-ray sources was employed to chemically characterise the surface of the specimens. One of the sources incorporates a monochromator (AI anode) and the second is an achromatic dual anode design (AI and Mg). For the present study, most spectra were collected using the Mg anode (1353.6 eV), however, in selected cases, the AI anode (1486.6 eV) was also employed to determine if particular element photolines (i.e., ones typically masked by Mg K,~ X-ray induced Auger lines) were present. XPS is capable of performing analysis of the outermost few atomic layers of solid samples, thereby providing surface analysis. Figures 6 to 12 give a summary of the XPS study in a comparative way. These Figures were prepared using selected elements considered relevant for the cyanidation experiments and for the pre-leaching with lead nitrate. The approximate analyses reported in these figures were calculated from surface analyses reported by XPS, but a small correction was applied to compensate for carbon contamination which comes either from the air, water and/or the vacuum grease in the desiccators. The carbon on the surface was adjusted to the carbon values found at 50 to 100 A. depths, whereas the proportion of the other elements at the surface was maintained constant. Despite this correction, the major elements on the surface are carbon and oxygen in carbon compounds from contamination. Because of this, it is important to note that the data in these Figures should be used in a comparative manner, for example sulphur on the surface of a given sulphide present in cyanidation with no pre-leaching compared with the experiment with pre-leaching.
Discussion The use of a gold foil was selected for analytical purposes. The consequence is that the passive layer related to the formation of reaction product on the substrate represents a smaller ration than it would be on gold
Effectof lead nitrateon cyanidationof gold ores
1271
grains. Major components of the surface analysis by XPS are gold (25-50%), carbon and oxygen from contamination with the aqueous or from manipulations (40-65%).
2.50
2.00
~"
1.50
z o
ell 7-
1.00
0.50
0.00
Chalcopyrite
Fig.6
Pyrite
Pyrrhotite
Effect of lime and lead nitrate pre-leaching on the consumption of sodium cyanide in the cyanidation experiments with gold foil and in the presence of various sulphides. Pre-leaching conditions, where applicable: 8 ppm 02, pH 11.5, 100 g/t Pb(NO3)2, 2 hours. Cyanidation conditions: 8 ppm 02, pH 11.5,500 g/t NaCN, 4 hours.
The effect of lead nitrate on the surface of the sulphide is clearly displayed by the results with pyrite. Figure 7 shows that the predominant iron species at the surface of pyrite after the cyanidation with n o lead pre-leaching are as (Fe÷3) oxy-forms (i.e. as iron (3) oxides or hydroxides). Whereas, the predominant iron species at the surface of the pyrite after lead pre-leaching and cyanidation are as sulphides.
_• 30
-
20
-
10
-
Pb.
]
29.7
.u_ E o
<
Pb
Pb
I Iron sulphides
Fig.7
I Fe-oxy forms
110-3 Pb-oxy forms
Main differences of pyrite surfaces after cyanidation with no pre-leaching treatment and with lead nitrate pre-leaching treatment ( marked "Pb").
1272
G. Deschenes et al.
The effect of lead nitrate on the pyrrhotite surface has some similarities with the case of chalcopyrite. Figure 8 shows that also the pyrrhotite surface both after cyanidation with no lead nitrate pre-leaching and pre-leaching followed by cyanidation have iron predominantly as oxy-forms species. The iron sulphide on the pyrrhotite surface after the lead pre-leaching is somewhat higher than on the pyrrhotite after cyanidation with n o pre-leaching. Also the pyrrhotite surface after the pre-leaching followed by cyanidation, contains lead hydroxide. This indicates that, as in the case of chalcopyrite, the pre-leaching is slowing down the surface reaction of the sulphur with the cyanide solution, but is not as effective in protecting the surface of pyrrhotite as in the case of pyrite. Figure 6, shows that, under identical pre-leaching, the effect is string in reducing the amount of cyanide used in presence of pyrite; whereas, this effect is not significant in the case of the cyanidation in the presence of chalcopyrite and pyrrhotite. XPS analyses were performed at the surface and at different depths up to 500 ~. As expected, the layers with lead on the pre-treated sulphides are not discrete; i.e. they do not end abruptly, but are diffuse. The maximum lead contents occur at the surface, smaller quantities of lead were found at depths of up to 500 A.. Pb
30.0
A
._u E
20.0
t
0.0 Iron sulphides
Fig.8
Fe-oxy forms
I o.4 Pb-oxy forms
Main differences of pyrrhotite surfaces after cyanidation with no pre-leaching treatment and with lead nitrate pre-leaching treatment ( marked "Pb").
At the pre-leaching stage, part of the lead gets to the surface of the pyrite where it is found predominately as an oxy-form followed by lead sulphate. XPS, in practice, can not easily discriminate between the lead oxy-forms, (e.g. PbO, Pb(OH)2). However, other researchers (Mussatti e t al. 1997) have found by using computer-aided thermodynamic calculations that lead hydroxides are the species thermodynamically predominant at the conditions of the cyanidation. Therefore, the lead oxy-form referred in all the XPS tables must be the lead hydroxide Pb(OH)2. XPS found much less lead on the surface of the "fresh" pyrite and on the surface of the pyrite after cyanidation with n o pre-leaching. This lead is mainly as sulphate followed by lead hydroxide. The source of this lead is the small quantity of galena (-0.2 wt.%, Table 3) in the pyrite. The results indicate that lead hydroxide is formed on the surface of the pyrite. This yields an inhibiting layer at the pyrite surface, which slows down its reaction with cyanide. The lower concentration of sulphur at the surface of the pyrite, with no lead pre-leaching, indicates that sulphur reacted with cyanide. This reaction competes for the cyanide required for gold dissolution, increases the consumption of cyanide and reduces leaching kinetics. The effect of lead nitrate on the surface of chalcopyrite is subtler than in the case of pyrite. Figure 9 shows that the chalcopyrite surfaces both after cyanidation with no lead nitrate pre-leaching and pre-leaching followed by cyanidation have similar contents of iron oxy-forms. However, it clear that the surface of the chalcopyrite after lead pre-leaching and cyanidation contains more iron sulphides than the surface of the
Effect of lead nitrate on cyanidation of gold ores
1273
chalcopyrite after cyanidation with n o lead pre-leaching. It is interpreted that the pre-leaching, is less effective in inhibiting the reaction between chalcopyrite and cyanide than in the case of pyrite. Because in the case of chalcopyrite the predominant iron species is as oxy-forms. Whereas in the case of pyrite, the predominant iron species is as sulphides. This agrees well with the effect of the pre-leaching on the consumption of cyanide for these cyanidation experiments. Figure 6 shows that, under identical conditions, pre-leaching has a strong effect in reducing the amount of cyanide used in presence of pyrite; whereas, it is not efficient to reduce cyanide consumption for chalcopyrite. Probably a more aggressive pre-leaching would be required to have an effect.
20.0 S
15.0
-
10.0
-
m
Pb
18.7
A
E
Pb
o
5.0
-
Ph
0.0 Iron sulphides
Fig.9
Fe-oxy forms
Pb-oxy forms
Cu
Main differences of chalcopyrite surfaces after cyanidation with no pre-leaching treatment and with lead nitrate pre-leaching treatment ( marked "Pb").
Figures 10 to 12 give the approximate analysis for the surfaces of the gold foils. It is possible to observe that there is sulphur on the surface of most the gold foils. XPS indicated that the sulphur is present as sulphides or polysulphides of gold. The sulphur on the surface may passivate the gold and therefore could be detrimental for the cyanidation acting as a barrier. For the case of the cyanidation done in the presence of pyrite, there is more sulphur on the gold foils with n o lead pre-leaching than with pre-leaching (Figure I0). The lead nitrate pre-leaching reduces the dissolution of the sulphur from the pyrite. Some sulphur, instead of forming thiocyanate, is adsorbed on metallic gold. Therefore the less sulphide dissolution, the less sulphur adsorption occurs on gold. A similar phenomenon is seen in the case of the cyanidation in the presence of pyrrhotite (Figure 11). However, this is not observed for the case of chalcopyrite. In presence of lead addition, the gold foils from cyanidation with pyrite and chalcopyrite showed surface concentrations of iron, most of the time as iron oxy-forms. The iron content is lower at the outer most part of the layer than at 50 A depth, probably indicating a higher affinity of the iron for gold than for the sulphur containing part of the layer. The iron oxy-forms on the surface of the gold may be beneficial for the cyanidation. Only the gold foils from cyanidation in the presence of chalcopyrite after lead nitrate pre-leaching show a very thin layer (<50/~) containing lead, most probably as lead hydroxide. No lead was detected on the gold foils from cyanidation in the presence of pyrite and pyrrhotite. This indicates that lead has more affinity for pyrite and pyrrhotite than for the gold. No lead was detected on the gold foils from cyanidation in the
1274
G. Deschenes et al.
presence of pyrite and pyrrhotite. Further investigations under progress, using similar experiments, have confirmed that there is no lead on the gold foils from cyanidation experiments in the presence of pyrrhotite with lead nitrate pre-leaching but there is lead for the case where realgar is the sulphide present. In all, cases, it is clear that the formation of lead or lead alloys on the surface of the gold is not as strong and obvious as the case where gold is in contact with lead nitrate in cyanide solutions in the absence of sulphides.
/I
/ 6.0
r
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-
2.0
-
5.3
A
u
E
,9o
,<
Pb Pb
. _
(
s
I
0.0 / / ~ J
"I
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"'l
Fe
/
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Main differences of gold surfaces after cyanidation in presence of pyrite. Lead nitrate pre-leaching treatment marked "Pb".
3.0 -J 2.4 2.0
-
t.0
-
A
,_u E <
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.
0.0 /
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t S
Fig. 11
.
0.1
.
.
.
.
.
.
.
.
.
I
.
.
.
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.
I Pb
Main differences of gold surfaces after cyanidation in presence of pyrrhotite Lead nitrate preleaching treatment marked "Pb".
Effect of lead nitrate on cyanidationof gold ores
1275
3.0
A
.-e
2.0
o
, m
E
o <
1.0 Pb
Pb
Pb
0.0
S Fig. 12
Fe
Pb
Cu
Main differences of gold surfaces after cyanidation in presence of chalcopyrite. Lead nitrate preleaching treatment marked "Pb".
PART 3---CYANIDATION STUDIES Experimental The ore samples were air-dried, homogenised and ground to 76%, 82%, and 90% -74~tm. The particle-size distribution used was similar to those typical of plant operation. The samples were cut into 500 or 1000 g lots using a rotary separator. Table 4 gives the assay for gold, silver and sulphur and the amount of cyanide consumers in the samples. The second sample has some unleachable gold, which is mostly in arsenopyrite. TABLE 4 Partial chemical and mineralogical analyses of the gold ores Ore Chemical analysis
Cyanicide (%)
1 4.21 0.90 1.53 0.4 3.1 tr.
Au (g/t) Ag (g/t) S (%) Pyrite Pyrrhotite Chalcopyrite Arsenopyrite tr. traces, less than 0.05%
2 4.63 0.63 1.29 2.7 tr. tr. 3.0
3 6.72 18.75 3.23 5.0 1.8 0.6 tr.
Lime, sodium cyanide, lead nitrate and oxygen were all certified reagent grade chemicals and plant water was used in the cyanidation tests. The gold leaching cells was made of glass and had a one litre or two litre capacity. The details of the experimental procedure are described in a previous report (Desch~nes and Fulton 1998). Air or a mixture of air and oxygen was injected to maintain the dissolved oxygen (DO) concentration constant, and the pH was controlled by lime addition. Water was added to maintain the pulp density at a constant value. In the case where lead nitrate was added during pre-leaching, it was introduced immediately after the start of mixing. No filtering was done after this stage and the cyanidation used the
1276
G. Deschenes et al.
same pulp. The cyanidation tests had duration of 24 h or 30 h. At the end of the experiment, the filter cake was washed with 1000 ml of distilled water. The cake wasdried, homogenized, sampled and analyzed for precious metals by fire assay. The leach solutions and wash solutions were titrated for free cyanide with silver nitrate and rhodamine as an indicator, and then assayed for gold and base metals (Fe, Cu) by atomic absorption spectroscopy. The calculation of cyanide consumption was based on total recycling of the leach solution. Table 5 presents the optimum experimental conditions and results for these cyanidation studies. The effect of the different leaching parameters studied is detailed in previous publications (Desch~nes and Fulton 1998, Desch~nes et al. 1999a, Desch~nes et al 1999b). Analysis of the data indicates that the lead nitrate addition strategy is a function of the mineralogical composition of the ore. The first gold ore, with only 0.4% pyrite and 3.1% pyrrhotite does not require a pre-leaching before cyanidation. Direct addition to cyanidation of a small amount of lead nitrate (50 g/t) reduced the cyanide consumed by 15% and increased the initial leaching kinetics (Figure 13). T A B L E 5 L e a c h i n g p a r a m e t e r s a n d p e r f o r m a n c e for the free m i l l i n g s u l p h i d e gold ores
Item \ Ore Grindin~ Pre-leaching
(% -74~tm) Duration (h) pH Dissolved oxygen (mg/L 02) Lead nitrate (~/t) Duration (h) pH Dissolved oxygen (m~/L 02) Lead nitrate (~/t) Free cyanide (ppm NaCN) Gold extraction (%) Tail (~/t Au) NaCN (kg/t)
Cyanidation
Efficiency
90
82
• --
76
- 2-6
16
---
- 11.8
11.3
----24 10.0
-8 - 50 24-30 11.8
8 100 24 11.3
8 50 380 94.8 0.22 0.22
0 250 90.0 0.48 0.33
0 360 95.8 0.28 0.50
100 90 80 70
o
60
~
50
<
No Pb(NOs)2
40 30 20 10 0
i 0
i
I 6
i
,
I
i
12 Time
,
I 18
i
, 24
(h)
Fig. 13 Effect of lead nitrate addition of the leaching of a gold ore with pyrite and pyrrhotite; Ore #1.
Effectof lead nitrateon cyanidationof gold ores
1277
The second gold ore, with 2.7% pyrite and 3.0% arsenopyrite, requires a very short pre-leaching time (between two and six hours) and 25-50 g/t lead nitrate. Improvement of the leaching kinetics resulted in a 43% reduction of the retention time. As illustrated in Figure 14, 89% gold extraction can be reached in 30 hours while the plant used previously 53 hours. At the plant scale (New Britannia Mine), the addition of lead nitrate also increased the overall gold extraction by 1.9%, from 89.2% to 91.1%. The cyanide consumption was reduced from 0.69 kg/t to 0.44 kg/t. The increment in gold recovered and the reduction in the cyanide consumption represented an additional gross revenue of $CND2.2 millions.
100 89%
90
50 ~ Pb(NO3)2
80
84%
70 ¢ 0
60 50
<
40 30 20 t0 0 0
6
12
18
24
30
Time (h)
Fig. 14 Effect of lead nitrate addition of the leaching of a gold ore with arsenopyrite and pyrite; Ore #2. The mineralogical analysis of the third ore indicated 5.0% pyrite, 1.8% pyrrhotite and 0.6% chalcopyrite. Without oxygen enrichment of the pulp, a long pre-leaching period with lead nitrate was required because of the high concentration of chalcopyrite and to a lesser extent the high concentration of pyrite in the ore. The 16-hour pre-leaching period reduced the cyanide consumption by 38% (0.81 kg/t to 0.50 kg/t) and increased gold extraction by 4.2% (from 91.6% to 95.8%) for a 24 hour cyanidation. Figure 15 shows the effect of lead nitrate addition in the pre-treatment on the gold leaching kinetics. Modifications to the plant practice allowed for an annual additional gross profit of $CAN375,000 in 1998.
100 90
100 g/t Pb(NO3)z
80 70 60 50 <
40 30 20 lO o
i
0
Fig. 15
i
I
6
r
,
I
12 Time (h)
,
,
I
18
i
i
24
Effect of lead nitrate addition of the leaching of a gold ore with pyrite, pyrrhotite and chalcopyrite. Ore #3.
1278
G. Descheneset al.
Gold ore #3 has the highest cyanide consumption with 0.50 kg/t. The presence of chalcopyrite, 0.6%, is mainly responsible for the high reactivity of this ore with cyanide. Between 70-75% of the cyanide consumed is related to complexation with copper. Gold ore #2 is second for cyanide consumption with 0.33 kg/t. The second gold ore generated higher cyanide consumption than the first gold ore that has 3.1% pyrrhotite, 0.4% pyrite and a cyanide consumption of 0.22 kg/t. It was found in a previous study that pyrite is more detrimental than pyrrhotite for cyanide consumption (Desch~nes et al. 1999c). Particle size is also a factor in this case. The first gold ore was ground to 76% -74~tm compared to 90% - 7 4 p m for the second one. Therefore, fine grinding of a sulphide-containing ore increases cyanide consumption. The rate constant for the dissolution reaction for gold is a function of the formation of a passive layer. Figures 13-15 show that the passive layer has a significant influence in the initial stages of leaching. Crundwell and Godorr (1997) developed a model based on a shrinking particle equation. This model considers the electrochemical nature of the mechanism and the effect of a passive layer. However, to take into account the important influence of the passive layer in the initial stages of leaching, his equation would have to be modified. The critical concentration of free cyanide, which is defined as the minimum concentration of free cyanide to ensure an efficient extraction of gold, is influenced by the presence of the passive layer, in spite of the fact that lead nitrate diminishes its effect. Significant decrease of the leaching rates were observed when the free cyanide concentration was at 170 ppm, 180 ppm and 270 ppm NaCN for ores 1, 2 and 3 respectively. The real free cyanide concentration in the leach solution of the third ore is much lower than indicated because of the interference by copper cyanide during titration of free cyanide with silver nitrate. The fall in the gold extraction was also the highest for the 3rd ore with 30%.
CONCLUSIONS In a cyanide solution, lead nitrate, lead sulphide and lead sulphite react with gold to form AuPb2, AuPb3, and metallic lead and clearly accelerates the gold dissolution. The nature of the sulphide minerals affects the way that lead or lead alloys will be formed on the gold surface. XPS did not find any lead on the surface of gold in the presence of pyrite or pyrrhotite but found a very thin layer (<50 A) in presence of chalcopyrite. Further investigation is required to study the effect of the presence of other sulphides. In presence of sulphide minerals, sometimes lead does not report on gold because of its high affinity for sulphide minerals (competing reactions). Pyrite, chalcopyrite and pyrrhotite showed different reaction mechanisms with lead nitrate. Lead nitrate forms a hydroxide layer on pyrite particles which reduces the reaction rate with cyanide. The dissolution of pyrite generates a sulphur layer on gold. This layer is less important in presence of lead. The effect of lead nitrate is subtler for chalcopyrite and pyrrhotite because it was less effective to retard the reaction of sulphides with cyanide and the reaction of iron with oxygen. For gold, the addition of lead nitrate has the same effect with pyrrhotite than with pyrite; inhibiting partially the formation of the sulphur layer. This was not observed for the gold in the chalcopyrite system. The results of the electrochemical study and the study of sulphide minerals correlate well with the cyanidation case studies. The strategy of lead nitrate addition, to be optimal, should be a function of the mineralogical composition of the ore. For a gold ore with a low sulphide content, lead nitrate addition can be performed directly to the cyanidation. A gold ore with a higher sulphide content, with a low sulphide content but finely ground, or with a significant amount of copper sulphide minerals requires a pre-leaching with a lead nitrate addition. The formation of a passive layer on gold particles has a significant influence in the initial stages of leaching. The addition of lead nitrate notably decreased its inhibiting effect. Improvement of the leaching rate substantially increased the throughput. In one case study, the plant was able to increase its throughput 41%, increase its gold extraction by 4% and reduce its cyanide consumption by 37%. The additional gold extraction represents additional revenues of $CND1.9 Million and cyanide reduction is $CND0.32Million.
Effect of lead nitrate on cyanidation of gold ores
1279
ACKNOWLEDGEMENTS
The authors wish to thank CANMET for permission to publish this paper and Dr. Allen Pratt for his editorial comments. REFERENCES
Arnold, R.G., Range in composition and structure of 82 natural terrestrial pyrrhotites. The Canadian Mineralogist, 1967, 9, 31-50. Chimenos J.M., Segarra M., Guzman L., Karagueorguieva A. and Espiell F., Kinetics of the reaction of gold cyanidation in the presence of thallium(I) salt. Hydrometallurgy, 1997, 44, 269-286. Crundwell F.K. and Godorr, S.A, A mathematical model of leaching of gold in cyanide solutions. Hydrometallurgy, 1997, 44, 147-162. Desch~nes G. and Fulton, M., Improving cyanidation of a sulphide ore by using an efficient pre-leaching. In Proc. Int. Symposium on the Recovery of Gold. Montreal, CIM General Annual Meeting, May 1998 (in press). Desch~nes, G., Fulton, M. and Lafontaine, M., Assessment and control of the gold leaching parameters at Kiena Mines. In Proc. Symp. Control and Optimization in Minerals, Metals and Materials Processing, MetSoc/CIM, 38 th Ann. Conf. Metallurgists, 1999, 469-487. Desch~nes, G., Fulton, M., Jean P. and Healey, S. , Improvement of cyanidation at New Britannia Mine. In Proc. Randol Gold and Silver Forum'99, Denver. 1999, 149-154. Desch~nes, G., Rousseau, M., Tardif J., and Prud'homme, P.J.H., Effect of the composition of some sulphide minerals on cyanidation and use of lead nitrate and oxygen to alleviate their impact. Hydrometallurgy, 1999, 50, 205-221. Fink C.G. and Putman G.L., The action of sulphide ion and of metal salts on the dissolution of gold in cyanide solutions. Mining Engineering, Transactions AIME, 1950, 187,952-955. Jeffrey M.I., Ritchie I.M., and LaBrooy S.R. , The effect of lead on the electrochemistry of gold: myth or magic. Electrochemical Proceedings, 1996, 96--6, 284-295. Jin S., May Oliver, Ghali E. and DeschSnes G., Investigation on the mechanisms of the catalytical effect of lead salts on gold dissolution in cyanide solution. In Proc. Third International Conference on Hydrometallurgy, ICHM'98, (Yang Xianwan, Chen Qiyuan and He Aiping eds.), International Academic Publishers, Beijing, China,. 1998, 666-679. Lehmann M.N., Kaur P., Pennifold R.M. and Dunn J.G., A comparative study of the dissolution of hexagonal and monoclinic pyrrhotites in cyanide solution. Hydrometallurgy, 2000, 5, 255-273. Liu C.Q. and Yen, W.T., Effects of sulphide minerals and dissolved oxygen on the gold and silver dissolution in cyanide solution. 1995, Minerals Engineering, 8, 1/2, 111-123. Mclntyre J.D. and Peck W.F. Jr., Electrodeposition of gold - - depolarization effects induced by heavy metal ions. J. Electrochemical Society, 1976, 123, 1800-1813. Mussatti D., Mager J. and Martins G.P. Electrochemical aspects of the dissolution of gold cyanide electrolytes containing lead. Aqueous Electrotechnologies: Progress in Theory and Practice (D.B. Dreisinger, ed.), The Minerals, Metals & Materials Society, 1997, 248-265. Rose T.K. and Newman, W.A.C., The Metallurgy of Gold, 2 "a edition, C. Griffin and Cy Ltd, London. 1898.
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